GLOSSARY

This glossary contains an extensive set of terms associated with AFM/SPM.

Acoustic mode An implementation of intermittent contact mode AFM in which the cantilever is driven by a piezoelectric actuator. The liquid cell utilized is the standard liquid cell used for all in-liquid AFM applications.

AFM image A high-resolution image acquired via the use of an atomic force microscope. The most common type of AFM image is one that displays the topography of a sample surface.

Alternating current mode (AC Mode) A generic name for an oscillatory probe technique of which patented Tapping Mode is a subset. An AC-like signal is used to drive the piezoelectric actuator (or to drive the MAC Mode coil) to produce a non-DC response of the cantilever to the drive signal.

Amplitude image An image in intermittent contact mode AFM that is generated by determining the difference between the measured amplitude and the setpoint value (a difference referred to as the error signal). This image is usually displayed and captured side-by-side a topography image.

Amplitude modulation AFM (AM-AFM) See intermittent contact mode AFM.

Artifact Any of several kinds of errors that could potentially appear in an AFM image, perhaps rendering it unusable.

Atomic force microscope (AFM) Also referred to as a scanning probe microscope, this scientific instrument works by bringing a cantilever tip in contact with a surface to be imaged. A repulsive force from the surface applied to the tip bends the cantilever upwards. The amount of bending, measured by a laser spot reflected onto a split photodetector, can be used to calculate the force. By keeping the force constant via a feedback servo while scanning the tip across the surface, the vertical movement of the tip follows the surface profile and is recorded as the surface topography.

Atomic force microscopy (AFM) A high-resolution imaging technique that can resolve features as small as an atomic lattice in real space. It allows researchers to observe and manipulate molecular- and atomic-level features. Application areas include life science, materials science, electrochemistry, polymer science, biophysics, nanotechnology, and biotechnology. Also referred to as scanning probe microscopy.

Bias voltage The voltage applied between the tip and the sample in scanning tunneling microscopy or scanning tunneling spectroscopy. It is also used for electrochemistry studies and current sensing AFM.

Cantilever A key component in an atomic force microscope. This flexible lever’s physical dimensions, as well as the material from which it is made, should be selected based on intended usage.

Closed-loop scanner An AFM scanner that uses closed-loop sensor feedback to correct for nonlinearity in the piezo scanning mechanism.

Contact mode AFM In this mode, also known as quasi-static mode AFM, interatomic van der Waals forces become repulsive as the AFM tip comes in close contact with the sample surface. The force exerted between the tip and the sample is on the order of about 0.1-1000nN. Under ambient conditions, two other forces besides van der Waals interactions are also generally present: the capillary force from a thin layer of water on the sample (condensed from the water in the air), as well as the mechanical force from the AFM cantilever itself.

Current sensing AFM This technique uses standard contact mode AFM along with ultrasharp AFM cantilevers coated with a conducting film to simultaneously probe conductivity and topography of a sample. By applying a voltage bias between the substrate and a conducting cantilever, a current flow is generated. This current can be used to construct a spatially resolved conductivity image.

Deflection The bending of the AFM cantilever from its free equilibrium position as a result of tip-sample interaction.

Detector signal The signal that comes back from a detector measuring the strength (and sometimes the polarity) of the interaction between the AFM probe and the sample surface. This signal can be used to construct an AFM image, without any feedback on the signal, or it can be used as the input of a feedback system, the output of which is used to construct an AFM image. In the latter case, the feedback system strives to maintain the value of the signal at a user-defined setpoint value.

Dynamic lateral mode AFM In this mode, the AFM cantilever is mechanically driven such that it twists about its long axis, usually at or near its fundamental torsional resonance frequency, typically several hundred kHz and into the MHz range. The tip then executes a rigid-pendulum-like rotational motion about the same axis, in a plane that is perpendicular to the axis. The apex of the tip dithers with a very small, typically sub-nanometer, amplitude.

Dynamic vertical mode force microscopy In this mode, the AFM cantilever oscillates, typically at frequencies in the kHz up to hundreds of kHz. The oscillations are such that the free end of the cantilever and the tip move along a gently curved trajectory on a plane perpendicular to the XY plane. The tip motion is therefore not strictly perpendicular to the XY plane, but the departure from perpendicularity is often negligible.

Electric force microscopy (EFM) This mode measures local electrostatic interaction between a conductive tip and a sample through Coulomb forces. Different areas of the surface may have different responses to the charged tip, depending on their local electrical properties. Variation in electrostatic forces can be detected in the change of oscillation amplitude and phase of the AFM probe.

Electrochemistry (EC) EC uses a potentiostat/galvanostat to control and monitor an electrochemical reaction at an electrified surface. Temperature and environmental control are often deemed important for these studies as well.

Environmental control The ability to precisely monitor and control a sample’s environment (including humidity levels, condensation/evaporation, oxygen levels, and gas concentrations) to enable in situ AFM studies.

Error signal A servo or feedback-loop input signal.

Force pulling In this force spectroscopy method, the AFM measures forces between the tip and the sample as the tip is pulled vertically away from the surface. The bond energies of the interactions between two molecules (intermolecular) or between different parts of a single molecule (intramolecular) can be extracted from these measurements.

Flow-through cell A liquid cell that allows solution to flow through a sample’s environment at a controlled rate, enabling the monitoring of real-time changes while exchanging solutions during in situ AFM studies.

Force spectroscopy A technique used to measure and sometimes control the polarity and strength of the interaction between an AFM tip and a sample. Functionalized tips may be utilized to study specific interactions of conjugated molecules. Some of the fastest growing applications of force spectroscopy involve a method commonly referred to as force pulling.

Force volume mode An example of a hybrid technique that combines force spectroscopy and imaging, either in contact mode AFM, or in intermittent contact mode AFM.

Free equilibrium position The position of an AFM cantilever when the cantilever is a sufficient distance from the surface to avoid tip-sample interaction forces.

Frequency modulation AFM (FM-AFM) One of the methods of operating an AFM in a dynamic vertical mode. In this technique, the frequency is changed in response to changes in the phase or amplitude response of the cantilever while the AFM tip is in contact with the sample. The amplitude variation and phase lag during the scan are measured.

Friction force The resistance between the sample surface and the AFM tip that causes a lateral bending (a torsional response) of the cantilever as the tip scans the sample. In friction force microscopy, it is necessary to decipher the variation in friction force from the topography.

Friction force microscopy See Lateral force microscopy.

Functionalized tip An AFM tip that has been intentionally exposed to chemical or biological species as part of a given experiment protocol.

Height In AFM topography images, the third dimension, Z, at a given X,Y coordinate pair, is the relative height of the sample surface at those coordinates. AFM height measurements are generally calibrated against height standards. These standard dimensions are often confirmed with methods other than SPM.

Higher harmonic imaging The use of higher resonance modes of the cantilever provides contrast different from that seen with fundamental amplitude and phase signals, allowing the collection of additional information about mechanical properties of the sample surface.

I-V curve Current vs. voltage. Local current-voltage characteristics of a sample can be measured via AFM or STM.

Intermittent contact mode AFM In this mode, the AFM cantilever’s oscillation amplitude, phase relative to the drive signal, and the sample’s surface topography are measured. Usually, the quantity that the feedback system strives to control is the cantilever’s oscillation amplitude. Also referred to as either amplitude modulation AFM or slope detection mode AFM.

Kelvin force microscopy (KFM) An AFM imaging technique that measures the potential of the surface at each in-plane position. The potential of the tip is matched to the potential of the sample by nulling the vibration amplitude of the cantilever due to a small applied AC bias with a DC offset. If the DC offset of the tip differs from the local surface DC potential, there will be a vibration at the frequency of the applied AC bias. Also known as surface potential imaging.

Lateral force microscopy (LFM) Also known as friction force microscopy. During contact mode AFM scanning, as the probe is dragged over a surface, changes in surface friction and topographic slope can cause the AFM cantilever to twist and thus create forces on the cantilever that are parallel to the plane of the sample surface. Such lateral forces cause lateral deflection of the cantilever, which is sensed by the photodetector and used to form a lateral force image.

MAC Mode III This advanced version of MAC Mode provides three lock-in amplifiers and allows single-pass imaging concurrent with Kelvin force microscopy and electric force microscopy. It also supports the use of higher resonance modes of the AFM cantilever.

Magnetic force microscopy (MFM) This mode measures magnetic structures/domains of a surface using a magnetic cantilever. As the magnetic tip scans, the interaction between the tip and the surface is greatly affected by the local magnetic properties. The variations in magnetic forces are measured in acoustic AC mode.

Molecular pulling See Force pulling.

Nanografting An AFM-based lithography technique that combines the displacement of selected resist molecules by an AFM tip and simultaneous adsorption of new adsorbate from the solution. It offers an effective means to fabricate organic thin film nanostructures with precise control over their location, size, geometry, terminal chemistry, and local environments.

Nanoindenting/nanoscratching The use of AFM to measure the mechanical properties of a given material by probing nanoscale indentations/scratches.

Nanomanipulation The use of AFM to effect a change in a material at the nanoscale, such as cutting and bending carbon nanotubes or stretching DNA.

Nanolithography The use of AFM to create patterned structures at the nanoscale. Techniques include nanografting.

Nanometer One billionth of a meter.

Nose cone AFM cantilever modules optimized for different imaging modes. A universal socket on a multipurpose scanner enables these modules to be changed quickly and easily.

Open-loop scanner An AFM scanner that does not have sensors for closed-loop feedback correction.

Oscillation The movement of the free end of the AFM cantilever and its tip as described in terms of amplitude, frequency, and phase.

Phase imaging This is a derivative imaging mode of dynamic vertical mode AFM. As the vertically oscillating AFM tip encounters regions of different composition, a change in phase, relative to the phase of the drive signal, is measured and recorded. This mode is especially useful for polymer research, and for electrical and magnetic property investigations, as in electric force microscopy and magnetic force microscopy.

Primary imaging modes There are at least five basic imaging modes widely in use today: scanning tunneling microscopy, quasi-static or contact mode AFM, dynamic vertical mode(s) of AFM, dynamic lateral mode AFM, and scanning near-field optical microscopy. The main feature that distinguishes the five modes from one another is the differing nature of their respective probe tip-sample interactions. Each of these five modes, in turn, enables a number of derivative imaging modes.

Probe tip Located at the free end of the AFM cantilever, the probe tip interacts with the sample surface. Different tip sizes, shapes, and materials continue to be introduced to the market.

Probe tip-sample interaction The interaction between an AFM probe tip and a sample can vary in several fundamental ways, including the type of interaction (electrical, mechanical, optical, or a combination of these), the time scales involved in the interaction, and the proximity of the sample to the probe tip.

Quasi-static mode AFM See Contact mode AFM.

Raster scan A line-by-line scan pattern in which the end point of a scan line in one direction (trace) is the starting point of the next scan line in the opposite direction (retrace), and so forth. The result is a subtle zigzag pattern that closely resembles a set of parallel lines.

Sample plate An AFM sample stage that can be used with a variety of sample-mounting options, including Petri dishes, liquid cells, glass microscope slides, and salt-bridge cells. Some sample plates offer temperature control.

Scan rate Along with setpoint and servo gain, one of the three most important parameters to change when optimizing the quality of an SPM image in a feedback mode. Faster scan rates require more aggressive feedback, which means higher feedback gain. Note that scan speed is also important. At different scan sizes, the tip velocity relative to the surface differs for the same scan rate.

Scanner A critical component in an atomic force microscope that houses the scanning elements as well as associated electronics. Common scanner types include open-loop and closed-loop AFM scanners, as well as STM scanners.

Scanning near-field optical microscopy (NSOM) This primary imaging mode relies on a combination of optical and mechanical interactions between the probe tip and the sample. It lends itself very well to spectroscopy in the traditional sense, but with a much enhanced lateral (X,Y) resolution.

Scanning probe microscope (SPM) See Atomic force microscope.

Scanning probe microscopy (SPM) See Atomic force microscopy.

Scanning probe spectroscopy See Spectroscopy modes.

Scanning tunneling microscope (STM) The predecessor of the atomic force microscope, this scientific instrument was invented in 1981 by G. Binnig and H. Rohrer, who subsequently shared the 1986 Nobel Prize in Physics.

Scanning tunneling microscopy (STM) This technique uses a sharp conducting tip and applies a bias voltage between the tip and the sample. When the tip is brought close to the sample, electrons can “tunnel” through the narrow gap either from the sample to the tip or from the tip to the sample, depending on the sign of the bias voltage. This tunneling current changes with tip-to-sample distance, decaying exponentially as distance increases, thus affording remarkably high precision in positioning the tip (sub-angstrom vertically and atomic resolution laterally). For the electron tunneling to take place, both the sample and the tip must be conductive.

Scanning tunneling spectroscopy The first spectroscopy technique that used an SPM, the energy levels of an object (for example, an atom or molecule) into and out of which a tunneling current flows are studied as the bias voltage across the tip and the sample changes in magnitude and in polarity.

Servo gain Along with setpoint and scan rate, one of the three most important parameters to change when optimizing the quality of an SPM image in a feedback mode. There is a limit to how much the servo gain can be increased; too much will result in problematic feedback oscillations that will show up in the image.

Setpoint Along with servo gain and scan rate, one of the three most important parameters to change when optimizing the quality of an SPM image in a feedback mode. The value of the setpoint reflects the desired strength of the signal at the detector, which in turn is related to the strength of the probe tip-sample interaction.

Slope detection mode AFM See Intermittent contact mode AFM.

Spectroscopy modes A class comprising several non-imaging techniques in which raster scanning in an imaging mode is disabled to allow the SPM to record the interaction between the sample and the probe tip at a given point in the sample plane (a given X,Y coordinate), while one or more parameters is changed by either stepping them in discrete quanta or ramping them at a rate under user control. Also known as scanning probe spectroscopy.

Temperature control The ability to heat and/or cool a sample for in situ AFM studies.

Tip See Probe tip.

Topography image This is the most common type of SPM image, usually created with atomic force microscopy or scanning tunneling microscopy. Because the AFM tip penetrates a harder sample surface less than it does a softer one, the tip is able to provide higher fidelity when following the height variations of a hard surface.

Tunneling current In scanning tunneling microscopy, the error/servo input signal is the tunneling current between the sample and a sharp metal tip (typically hundreds of pA to several nA) when a bias voltage (typically tens or hundreds of mV) is applied between the two. The magnitude of this current is extremely sensitive to (and varies exponentially with) the small gap that separates the nearest atoms between the STM tip and the sample surface.

Van der Waals forces A term used to describe a number of attractive or repulsive forces between atoms or molecules. In contact mode AFM, these forces become repulsive as the AFM tip comes in close contact with the sample surface.

Z-actuator This piezoelectric tube, flexure, or hybrid of the two is responsible for moving the AFM probe in the Z-axis.